1. Field of the Invention
The present invention relates to a capacitor mounting structure, and particularly to a capacitor mounting structure in which a resin case for housing a capacitor is mounted on a board surface of a printed board.
2. Description of the Related Art
JP-A-4-180211 discloses that a capacitor is housed in a resin case having a lower opening, a filler is filled between the capacitor and the resin case, the lower opening of the resin case is made to face a printed board, and the resin case is mounted on a board surface of the printed board.
This prior art describes that a stress is generated in a soldered portion of a lead from the capacitor because there is a difference in thermal expansion coefficient between the capacitor and the printed board or because the printed board is curved by heat at the time of solder dipping and has a displacement in the radial direction of a curve, and accordingly, a step of further applying heat to the solder to perform re-soldering is required after the solder dipping step in order to relieve the stress to return the printed board to the original state, in a case where a temperature change is large and the stress to the soldered portion is large, or in a case where the temperature change frequently occurs and the stress is repeatedly applied to the soldered portion, the solder can not resist the stress, and a crack occurs in the soldered portion, the crack in the soldered portion occurs in the soldered portion on the lower board surface of the printed board, the stress in the direction parallel to the board surface of the printed board has a greatest influence on the occurrence of the crack in the soldered portion, and the crack causes poor conduction of the capacitor.
This prior art discloses that a gap part of 3 mm or more is formed between the filler exposed from the lower opening of the resin case and the printed board, so as to mount the capacitor, and describes that the gap part of 3 mm or more can relieve the stress generated by the temperature change of the periphery of the attachment place of the capacitor so that the burden of the stress to the soldered portion of the extraction lead of the capacitor can be reduced.
In the prior art (JP-A-4-180211), FIGS. 2, 4 and 5, and the description in page 1, right column, line 11 to page 2, left column, line 10 are especially noted.
However, since the capacitor mounting structure disclosed in the prior art has such a structure that the self weights of the capacitor, the filler and the resin case are supported by the extraction lead and the bottom surface of the resin case being in contact with the upper board surface of the printed board, especially in a case of the capacitor used for a control unit mounted in a vehicle, a large vibration is repeatedly applied, and there is a fear that the extraction lead is broken by the vibration, and especially it has been difficult to mount a large capacitor. Besides, at the time of soldering, since fixation of the capacitor is not surely performed, a position shift or fall of the capacitor occurs by the vibration, and therefore, it has been necessary to ensure a sufficient gap around the capacitor on the upper board surface.